In conventional medical diagnostic imaging the object is to obtain an image of a patient's internal anatomy with as little X-radiation exposure as possible. The fastest imaging speeds are realized by mounting a dual-coated radiographic element between a pair of fluorescent intensifying screens for imagewise exposure. About 5 percent or less of the exposing X-radiation passing through the patient is adsorbed directly by the latent image forming silver halide emulsion layers within the dual-coated radiographic element. Most of the X-radiation that participates in image formation is absorbed by phosphor particles within the fluorescent screens. This stimulates light emission that is more readily absorbed by the silver halide emulsion layers of the radiographic element. For medical diagnostic imaging, film contrast typically ranges from about 1.8 to 3.2, depending upon the diagnostic application.
Crossover of light from one fluorescent screen to an emulsion layer on the opposite side of the support of the radiographic element results in a significant loss of image sharpness. Crossover is minimized, since this degrades image sharpness and creates the risk of the radiologist failing to observe a significant anatomical feature required for a proper diagnosis. At worst crossover in medical diagnostic elements can range up to about 25 percent, but in the overwhelming majority of medical diagnostic element constructions is less than 20 percent and, in preferred medical diagnostic radiographic elements, crossover is substantially eliminated.
Medical diagnostic X-radiation exposure energies vary from about 25 kVp for mammography to about 140 kVp for chest X-rays.
Examples of radiographic element constructions for medical diagnostic purposes are provided by Abbott et al U.S. Pat. Nos. 4,425,425 and 4,425,426, Dickerson U.S. Pat. No. 4,414,304, Kelly et al U.S. Pat. Nos. 4,803,150 and 4,900,652, Tsaur et al U.S. Pat. No. 5,252,442, and Research Disclosure, Vol. 184, August 1979, Item 18431.
Portal radiography is used to provide images to position and confirm radiotherapy in which the patient is given a dose of high energy X-radiation (from 4 to 25 MVp) through a port in a radiation shield. The object is to line up the port with a targeted anatomical feature (typically a tumor) so the feature receives a cell killing dose of X-radiation. In localization imaging the portal radiographic element is briefly exposed to the X-radiation passing through the patient with the shield removed and then with the shield in place. Exposure without the shield provides a faint image of anatomical features that can be used as orientation references near the target (e.g., tumor) area while the exposure with the shield superimposes a second image of the port area. The exposed localization radiographic element is quickly processed to produce a viewable image and to confirm that the port is in fact properly aligned with the intended anatomical target. During the above procedure patient exposure to high energy X-radiation is kept to a minimum. The patient typically receives less than 20 RADs during this procedure and exposure is limited to 10 seconds or less.
Thereafter, before the patient is allowed to move, a cell killing dose of X-radiation is administered through the port. The patient typically receives from 50 to 300 RADs during this step over a period of from 30 to 300 seconds. While the localization imaging procedure is relied upon to direct the high energy X-radiation beam through the port to the portion of the anatomy intended to be killed, there remains a possibility that the patient may have inadvertently shifted position between the localization X-radiation beam targeting and actual radiation therapy. Therefore, it is common practice to conduct radiation therapy with a portal verification radiographic element present. The verification element is exposed only within the area of the port. Within the port area anatomical feature can usually be identified to verify that the radiation therapy has, in fact, been targeted as intended.
A proposed portal radiographic element construction is disclosed by Sephton U.S. Pat. No. 4,868,399.Sephton does not disclose rapid access processing or a film construction capable of undergoing rapid access processing. Sephton further shows dual-coated structures to produce unsatisfactorily low levels of contrast. Sephton discloses portal localization, but not portal verification imaging.
Medical diagnostic imaging has in recent years learned to employ silver halide emulsions at silver coating coverages of less than 30 mg/dm.sup.2 by employing tabular grain emulsions. The high ratio of grain projected area to thickness allows high levels of silver image covering power to be realized, as first observed by Dickerson U.S. Pat. No. 4,414,304. The relatively high speeds of tabular grain emulsions render them unsuitable for use in use in portal imaging.
While lower silver coating coverages are in themselves advantageous in saving materials and facilitating rapid access processing, the low silver coverages have presented a problem in using commercially available rapid access processors, since they lack sufficient infrared density to be detected by the sensor beams used to sense the presence of radiographic film in rapid access processors.
Recent attempts to substitute high chloride silver halide emulsions for the high bromide silver halide emulsions most commonly employed in radiographic imaging have compounded the problem. Silver chloride exhibits a significantly lower refractive index than silver bromide and therefore creates lower specular densities when otherwise comparable grains are present at the same coating coverages. When coating coverages are less than 30 mg/dm.sup.2, the problem of detecting the presence of radiographic elements is compounded.
Harada et al U.S. Pat. No. 5,260,178 has noted that with low silver coating coverages in radiographic elements, it is impossible for sensors that rely on the scattering of near infrared sensor beams by silver halide grains to sense the presence of the film in the processor. The solution proposed is to incorporate an infrared absorbing dye. Instead of reducing specular density by scattering near infrared radiation, the dye simply absorbs the near infrared radiation of the sensor beam. During processing the dye is deaggregated to shift its absorption peak. In the later stages of processing the density of developed silver is relied upon for interrupting sensor beams, which is the conventional practice.
The difficulty with the Harada et al solution to the problem of insufficient silver halide grain coating coverages to activate infrared sensors is that it relies on the addition of a complex organic material--specifically a tricarbocyanine dye that must have, in addition to the required chromophore for near infrared absorption, a steric structure suitable for aggregation and solubilizing substituents to facilitate deaggregation. The dyes of Harada et al also present the problem of fogging the radiation-sensitive silver halide grains when coated in close proximity, such as in a layer contiguous to a radiation-sensitive emulsion layer. Simply stated, the "cure" that Harada proposes is sufficiently burdensome as to entirely offset the advantage of reduced silver coating coverages, arrived at by years of effort by those responsible for improving films for producing silver images in response to rapid access processing. Thus, Harada's film structure modification is not a problem solution that has practical appeal.